This invention relates to standing wave linear accelerators and, in particular, relates to standing wave linear accelerators for use in medical radiotherapy systems.
Radiation therapy involves delivering a high, curative dose of radiation to a tumor, while minimizing the dose delivered to surrounding healthy tissues and adjacent healthy organs. Therapeutic radiation doses may be supplied by a standing wave linear accelerator that is configured to generate a high-energy (e.g., several MeV) electron beam. In an electron mode of operation, the electron beam may be applied directly to one or more therapy sites on a patient. Alternatively, in a radiation mode of operation, the electron beam may be used to generate a photon (e.g., X-ray) beam that may be applied to the patient. The shape of the radiation beam at the therapy site may be controlled by discrete collimators of various shapes and sizes or by multiple leaves (or finger projections) of a multi-leaf collimator that are positioned to block selected portions of the radiation beam. In this way, the radiation beam may be contained within the boundaries of the therapy site, whereby healthy tissues and organs that are located beyond the boundaries of the therapy site may be protected against exposure to the radiation beam.
In general, a standing wave linear accelerator includes a particle source (e.g., an electron gun) that directs charged particles (e.g., electrons) into an accelerating cavity. The charged particles travel through a succession of accelerating cavities, in which the particles are focused and accelerated by an electromagnetic (RF) field that is applied by an external RF source (e.g., a klystron amplifier or a magnetron oscillator). The charged particles typically are formed and compacted into bunches by the initial accelerating cavities that are traversed by the charged particle beam. Bunching the charged particles increases the number of electrons available for acceleration at the fundamental resonant frequency of the accelerating cavities. This feature allows the charged particle source to operate at lower power levels and improves the overall efficiency of the system.
The invention features a standing wave linear accelerator with a prebunching section and an accelerating section that are formed into a unitary accelerating structure. The prebunching section is configured to group charged particles into bunches by velocity modulation of the charged particle beam. The accelerating section has a plurality of inter-coupled resonant cavities, including an input cavity that is coupled to the prebunching section and an output cavity.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The prebunching section of the unitary accelerating structure preferably is configured to group charged particles into bunches without net charged particle acceleration.
The accelerating section of the unitary accelerating structure preferably incorporates a bunching section for accelerating and further grouping charged particle bunches that are received from the prebunching section. The accelerating section of the unitary accelerating structure may be characterized by a fundamental resonant frequency, and the bunching section may be configured to further group charged particle bunches that are received from the prebunching section for more efficient modulation at the fundamental resonant frequency.
The system may include a power coupling circuit that is configured to enable independent phase and amplitude adjustment of rf energy that is injected into the prebunching and accelerating sections of the unitary accelerating structure. The prebunching and accelerating sections of the unitary accelerating structure may be coupled to the same source of rf energy. A directional coupler may be configured to apportion rf energy from the rf source between the prebunching section and the accelerating section.
In one embodiment, the prebunching section of the unitary accelerating structure includes a single resonant cavity. The resonant cavities of the accelerating section may be inter-coupled by side cavities.
A drift tube may be formed between the prebunching and accelerating sections of the unitary accelerating structure. A charged particle source may be coupled to the prebunching section of the unitary accelerating structure. In some embodiments, the charged particle source is a low-voltage electron gun.
In another aspect, the invention features a system for generating a therapeutic beam that includes a charged particle source, a standing wave linear accelerator that is formed from the above-described unitary accelerating structure, and a system that is configured to shape the therapeutic beam for delivery to a therapy site on a patient.
In some embodiments in accordance with this aspect of the invention, an x-ray target positioned to intercept a charged particle beam that is accelerated by the standing wave linear accelerator.
Among the advantages of the invention are the following.
In accordance with the invention, the reliability and operating efficiency of the linear accelerator are improved by forming the prebunching section integrally with the accelerating section. For example, such an integrated structure reduces concentration of electric fields at the interface between the prebunching section and the accelerating section that otherwise might limit the maximum power level at which the accelerator may be operated. In addition, such an integrated structure allows the accelerator to be manufactured more quickly and more cost effectively than if the prebunching section were formed as a separate unit and merely bolted or otherwise not integrally attached to the accelerating section.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.